I suffer from permanent AFib and my cardiologist says I should exercise, even with AFib. It is good to engage in physical exercise so long as you control the level to prevent over taxing your heart.
Because of the AFib my rest HR is high, in the 80s even with 150 mg of Metoprolol (BB) every day. So I don't have a lot of room for exercise. I try to keep my HR below 150 and hold any sustained HR to below 140. With these restrictions I can ride a bike on level or slight grade, same for walking.